The present invention generally relates to an electrochemical battery cell. More particularly, the present invention relates to battery cells with improved pressure relief vents.
Increasing the discharge capacity of electrochemical cells is an ongoing objective of manufacturers of electrochemical cells and batteries. Often there are certain maximum external dimensions that place constraints on the volume of a given type of cell or battery. These maximum dimensions may be imposed through industry standards or by the amount of space available into which the cells or batteries can be put. These dimensions limit maximum cell and battery volumes. Only a portion of the volume is available for the materials (electrochemically active materials and electrolyte) necessary for the electrochemical discharge reactions, because other essential, but inert, components (e.g., containers, seals, terminals, current collectors, and separators) also take up volume. A certain amount of void volume may also be necessary inside the cells to accommodate reaction products and increases in material volumes due to other factors, such as high temperature. To maximize discharge capacity in a cell or battery with a limited volume, it is desirable to minimize the volumes of inert components.
Electrochemical cells are capable of generating gas, during storage, during normal operation, and, especially, under common abusive conditions, such as forced deep discharging and, for primary cells, charging. Cells are designed to release internal pressure in a controlled manner. A common approach is to provide a pressure relief mechanism, or vent, which releases gases from the cell when the internal pressure exceeds a predetermined level. Pressure relief vents often take up additional internal volume because clearance is generally needed between the vent and other cell or battery components in order to insure proper mechanical operation of the mechanism.
Dimensions of consumer cylindrical alkaline batteries are specified in an international standard (International Electrical Commission Publication 60086-2). Such cells have a positive electrode containing manganese dioxide, a negative electrode containing zinc, and an alkaline aqueous electrolyte typically containing potassium hydroxide. They often have a cylindrical steel can that serves as the cell container, with the positive electrode (cathode) formed in a hollow cylindrical shape against the interior surface of the can. A gelled negative electrode (anode) is centrally disposed within the cylindrical cavity in the cathode. An ion-permeable, electrically insulating separator is placed between the anode and adjacent surfaces of both the cathode and the bottom of the can. Electrolyte solution is contained within both the anode and the cathode. The can, which is in direct contact with the cathode, serves as the cathode current collector. The open top portion of the can is closed with a closing element, typically including an annular polymeric seal. An outer cover is generally placed over the seal to serve as a negative terminal for the cell. In addition to closing the can, the seal also electrically insulates the negative terminal from the can. An anode current collector, usually in the form of a brass nail or wire, extends through an aperture in the center of the seal and into the anode within the cell. The end of the anode current collector on the outside of the cell makes electrical contact with the negative terminal. The bottom of the can may be flat, or it may be formed to have a central protruding nubbin that serves as the positive terminal of the cell. If the can bottom is flat, a separate metal cover is normally affixed to the can bottom as the positive terminal. A jacket, often an electrically insulating, adhesive film label, is generally placed around the side walls of the can. Cells may include additional features. For example, an inner cover or a bushing may be disposed between the seal and the negative terminal to provide a rigid member for maintaining a compressive seal between the seal and the surface of the can and/or anode current collector. In such cells the seal also typically contains a pressure relief vent. This feature usually includes a thinned area, which is designed to rupture when the internal pressure goes above a predetermined level. Examples of cells with seal designs of this type can be found in U.S. Pat. Nos. 5,227,261 and 6,312,850. However, this type of seal requires a relatively large amount of volume in order for the pressure relief vent to function as intended.
In order to increase the amount of active materials in cylindrical alkaline cells, more volume efficient cell designs have been developed. In some of these, the pressure relief vent has been taken out of the seal and put into either a metal cover outside the seal or into the bottom of the can. Many different designs are possible for pressure relief vents formed in metal plates, whether covers or can bottoms, for electrochemical cells. Some of these include raised ridges or depressed troughs, projecting outward or inward, respectively, from the surface of the vent-containing plate. Examples of cells with such pressure relief vent designs can be found in U.S. Pat. Nos. 3,831,822; 3,918,610; 4,484,691; 4,601,959; 4,789,608; 5,042,675; and 5,197,622. Each of these references suffers from one or more disadvantages. For example, they may rely on deformation of the plate at the ridges/troughs to concentrate stress in a weakened portion of the plate for the vent to open. This may require a relatively large deflection in the surface of the plate, which is counter-productive when maximizing the internal volume of the cell for active materials is an objective. Such designs may also be relatively complicated and difficult to manufacture, which can make precise, reliable control of the pressure at which the vent opens difficult.
Other pressure relief designs do not have ridges or troughs. Some of these have grooves, or scores, of reduced thickness in the surface of the vent-containing plate. These grooves create weak spots in the plate that are designed to tear or rupture when the pressure differential between the two sides of the plate becomes too great. A variety of such grooves can be used. For example, the groove may be in the form of: a circle, a partial circle, one or more curved lines, one or more straight lines, or two or more intersecting straight and/or curved lines. The grooves may be formed in the plate in any of a number of possible ways, such as by stamping, coining, scoring, and etching. It may be possible to combine the step of forming the pressure relief vent for an electrochemical cell with the process of making the component in which it is formed. The grooves may be formed when a cover or a can is formed, for example by stamping and/or drawing, using punches and dies, such as in a multiple-stage progressive die set or transfer press tooling. One or more steps of such a process can be modified and/or added to include the formation of the vent grooves.
Information relevant to previous attempts to address the above problems by using a cell design with a grooved pressure relief vent in a metal cell cover or the bottom of the can may be found in U.S. Pat. Nos. 3,074,602; 4,010,044; 4,256,812; 4,698,282; 4,803,136; 4,842,965; 6,265,101; 6,303,246; 6,346,342; and 6,348,281. Additional examples may be found in Japanese unexamined patent publication numbers 01-309,253 A; 09-139,197 A; 10-092,397 A; 11-213,978 A; and 11-250,886 A. However, each of these references suffers from one or more of the disadvantages described below.
Some grooved vent designs are expensive because they are complex and require more expensive tooling. Some designs require tooling that is more difficult to maintain. Others add unnecessarily to the difficulty and cost of manufacturing because the designs are not symmetrical, placing more stresses on the equipment and tooling, and increasing the frequency and cost of maintenance and replacement. Other grooved vent designs may be unsuitable for use in an electrochemical cell because, when the vent operates, a portion of the vent-containing plate may be ejected from the cell. Yet other grooved vent designs require too much clearance for the vent to function, making less internal volume available for active materials, or do not open a large enough area to relieve the internal cell pressure quickly enough to avoid damage or injury.
For the foregoing reasons, there is a need for a high-capacity electrochemical battery cell having a reliable, low-volume, cost-effective pressure relief vent.